Come the Night (Vampire/Werewolf Trilogy #3) by Susan Krinard

SOMETHING PRIMAL IN HIM CALLS TO HER...

Gillian Maitland is to marry a werewolf of her father's choosing -- ensuring the purity of their noble bloodline. Still, she can't forget Ross Kavanaugh, the man whose forbidden touch unleashed a passion she'd never known. And when Ross returns unexpectedly to England, he's no longer the man she remembers, but a hard-boiled ex-cop who harbours a dark secret.

The discovery that they have a son makes Ross even more determined to prove his worth to Gillian, despite being merely a quarter werewolf. Then a mysterious spate of murders casts him under a pall of suspicion, and torn between duty and desire, Gillian knows she must drive Ross away. Even as their hunger for each other grows by the hour...